ecipitation as due
to the cooling of a current of saturated air by contact with a colder
current, the assumption being that the surplusage of moisture was
precipitated in a chemical sense, just as the excess of salt dissolved
in hot water is precipitated when the water cools. The idea that the
cooling of the saturated air causes the precipitation of its moisture
is the germ of truth that renders this paper of Hutton's important. All
correct later theories build on this foundation.

"Let us suppose the surface of this earth wholly covered with water,"
said Hutton, "and that the sun were stationary, being always vertical in
one place; then, from the laws of heat and rarefaction, there would be
formed a circulation in the atmosphere, flowing from the dark and cold
hemisphere to the heated and illuminated place, in all directions,
towards the place of the greatest cold.

"As there is for the atmosphere of this earth a constant cooling cause,
this fluid body could only arrive at a certain degree of heat; and this
would be regularly decreasing from the centre of illumination to the
opposite point of the globe, most distant from the light and heat.
Between these two regions of extreme heat and cold there would, in every
place, be found two streams of air following in opposite directions. If
those streams of air, therefore, shall be supposed as both sufficiently
saturated with humidity, then, as they are of different temperatures,
there would be formed a continual condensation of aqueous vapor, in some
middle region of the atmosphere, by the commixtion of part of those two
opposite streams.

"Hence there is reason to believe that in this supposed case there would
be formed upon the surface of the globe three different regions--the
torrid region, the temperate, and the frigid. These three regions would
continue stationary; and the operations of each would be continual. In
the torrid region, nothing but evaporation and heat would take place;
no cloud could be formed, because in changing the transparency of the
atmosphere to opacity it would be heated immediately by the operation of
light, and thus the condensed water would be again evaporated. But this
power of the sun would have a termination; and it is these that would
begin the region of temperate heat and of continual rain. It is not
probable that the region of temperance would reach far beyond the region
of light; and in the hemisphere of darkness there would be found a
r